As a piezoelectric material, PZT (a PBZrO3—PbTiO3 solid solution), which is a ceramic material, has been used in many cases. However, since PZT contains lead, piezoelectric polymer materials having a low environmental impact and a high flexibility have been increasingly employed.
Currently known piezoelectric polymer materials are roughly classified mainly into the following three types, i.e., poled polymers typified by nylon 11, polyvinyl fluoride, polyvinyl chloride and polyurea, polyvinylidene fluoride (β type) (PVDF), and ferroelectric polymers typified by a polyvinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) (75/25).
However, since piezoelectric polymer materials are inferior to PZT in piezoelectricity, improvement in piezoelectricity is demanded. Thus, attempts have been made from various standpoints in order to improve the piezoelectricity of piezoelectric polymer materials.
For example, PVDF and P(VDF-TrFE), which are ferroelectric polymers, exhibit excellent piezoelectricity among polymers, and the piezoelectric constant d31 thereof is 20 pC/N or more. Film materials formed from PVDF and P(VDF-TrFE) are imparted with piezoelectricity by carrying out a drawing process in which polymer chains are oriented in a drawing direction; generating an electric field in a direction longitudinal to a film surface by providing different types of charges to a front side and a back side of the film by corona discharge or the like; and allowing permanent dipoles containing fluoride existing in a side chain of the polymer chains to be oriented in a direction parallel to the electric field direction. However, there has been a problem in practical use in that different types of charges, such as water or ions contained in air, are likely to adhere to the polarized film surface in a direction of canceling orientation; therefore, orientation of the aligned permanent dipoles may tend to cause a significant decrease in piezoelectricity over time.
PVDF is a material having the highest piezoelectricity among the piezoelectric polymer materials described above. However, since PVDF has a relatively high dielectric constant among the piezoelectric polymer materials, i.e., 13, the material has a small piezoelectric g constant (open-circuit voltage per unit stress), which is a value obtained by dividing the piezoelectric d constant by the dielectric constant. Further, although PVDF exhibits a favorable conversion efficiency from electricity to sound, a conversion efficiency from sound to electricity has yet to be improved.
In recent years, use of polymers having optical activity, such as polypeptide or polylactic acid, is attracting attention, in addition to the piezoelectric polymer materials described above. Optically active polymers are known to demonstrate piezoelectricity by carrying out a mechanical drawing alone.
Among the polymers having optical activity, piezoelectricity of polymer crystals, such as polylactic acid, originates from permanent dipoles of a C═O bond being present in a screw axis direction. In particular, polylactic acid, which has a small volume fraction of a side chain to a main chain and a high ratio of permanent dipoles per volume, is an ideal polymer among polymers having helical chirality.
It is known that polylactic acid, which demonstrates piezoelectricity by a drawing process alone, does not require a poling process, and that the piezoelectric modulus does not decrease over the years.
As described above, polylactic acid has various types of piezoelectric properties, and thus, piezoelectric polymer materials using various kinds of polylactic acids have been reported.
For example, a piezoelectric polymer material that exhibits a piezoelectric modulus of approximately 10 pC/N at room temperature, obtained by subjecting a molded article of polylactic acid to a drawing process, is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 5-152638).
Moreover, in order to obtain highly oriented polylactic acid crystals, performing a special orientation process referred to as a forging method to achieve a piezoelectricity of approximately 18 pC/N has also been reported (e.g., JP-A No. 2005-213376).